1. Field of the Invention
The present invention relates to optical devices incorporating prisms, and more particularly to an achromatic prism pair for the conversion of the elliptical beam from a tunable diode laser to a beam of circular cross section.
2. Description of the Prior Art
It has been recognized that the elliptically shaped output beam from a diode laser can be converted into the more desirable circular beam by the use of cylindrical lenses or, more desirably, a pair of prisms. The output beam from a diode laser, even when collimated by a lens, is generally in the form of an ellipse, typically one with an aspect ratio of 3 to 1. The elliptical shape does not lend itself to optimal performance of the associated system, which gave rise to various techniques for converting the elliptical beam into a round one. Obviously, the conversion device should not contribute to a deterioration in the other characteristics of the radiation or unduly complicate the optical system. The prism approach has been found to be a more practical approach than the use of lenses and the prism configuration may be selected to provide various fixed magnifications along one axis. Typical magnification ratios range from 2 to 6 times to accommodate lasers of various ellipticities. Such prisms are quite effective for the intended purpose at a fixed wavelength but possess serious shortcomings when the laser wavelength is varied.